Method and system for improved beacon acquisition performance with time slot and antenna sector reuse

ABSTRACT

A wireless multi-cell communication system, such as a time division duplex (TDD) system, includes at least one wireless transmit/receive unit (WTRU) and a plurality of base stations which broadcast beacon signals into a plurality of sectors of a cell of the system. Beacon channel acquisition performance is enhanced by reducing beacon channel mutual interference. Each of the base stations broadcasts a beacon signal into at least one of the cell sectors in a first time slot and to at least another one of the cell sectors in a second time slot different from the first time slot. The WTRU determines, at a particular position, which one of the beacon signals has the best signal quality, acquires the one beacon signal and establishes communications via the base station that generated the one beacon signal having the best signal quality.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/118,872 filed Apr. 29, 2005, which is a continuation of U.S. patentapplication Ser. No. 10/330,750 filed Dec. 27, 2002, which issued asU.S. Pat. No. 6,928,283 on Aug. 9, 2005, which claims the benefit ofU.S. Provisional Patent Application No. 60/412,273 filed Sep. 20, 2002,which are incorporated by reference as if fully set forth.

BACKGROUND OF THE INVENTION

The ability of a mobile or fixed wireless transmit/receive unit (WTRU)to acquire beacon channels in a fast and reliable manner is a keyfunctionality for a Third Generation (3G) Time Division Duplex (TDD)system. Beacon channels, also known as broadcast control channels(BCHs), transmit in a predefined slot within a particular frame of amulti-frame TDD system.

A beacon channel is always a BCH, but other channels such as the pagingindicator channel (PICH) or a paging channel (PCH) can also be used asbeacon channels. However, an important characteristic of a beaconchannel is that it must be transmitted at a fixed (high) reference powerlevel such that the beacon can be reliably received everywhere in thecell. This allows a WTRU to determine the pathloss estimate from theknown beacon channel. In the favored deployment of a TDD system, anunlimited number of base stations (BSs) may transmit their beaconsignals in the same time slot, allowing all the WTRUs in the coveragearea to measure all the beacons simultaneously. The WTRUs can thencompare the power received from each of the BSs in the coverage area andchoose to connect to the BS with the highest quality signal.

The WTRU acquires the beacon for system access information or to findother WTRUs for eventual cell handover or cell re-selection. Theadvantage of having all the neighboring BS beacons in the same timeslotalso leads to a disadvantage in the form of interference from all thehigh powered signals in the same time slot.

A beacon channel can be further defined as a special physical channelthat is sent at a fixed high reference power and uses a special reservedorthogonal variable spreading factor (OVSF) code, which is sent at leastonce per frame. A time slot (TS) that contains a beacon channel iscalled a beacon TS.

There are two common deployment scenarios for beacon time slots in TDDsystems. In the first scenario, only a single time slot is allocated asthe beacon TS of a cell. The single beacon TS containing the BCH isalways found at a specific time slot location TS(k) of the frame. In thesecond scenario, two time slots are allocated as the beacon TSs of acell. The BCH beacon information is sent at location TS(k) of the frameand the second beacon TS of the frame is located at TS(k+8). The secondbeam TS is known as the secondary beacon and it may contain otherdownlink channel information. The second scenario represents thepredominant deployment of a TDD system today.

TDD cells which operate in close geographical areas and on the samefrequencies need resource coordination and time synchronization in orderto achieve maximum system capacity and efficiency. The deployment of thebeacon TSs of TDD cells uses a scheme where the beacon channels of allneighboring cells are sent in the same time slot, thus requiring timealignment. The major benefit of time-aligned beacons is that it allowsWTRUs to simultaneously measure their neighboring cell BS and thecurrent serving cell BS. The WTRU may discover another BS with a bettersignal level and switch to that BS, thereby allowing the WTRU to reduceits transmitting power and preserve battery life. However, if thecoverage area has many BSs in close proximity, there is a strongpossibility that the time-aligned beacon TSs will lead to extremelydegraded BCH beacon acquisition performance for WTRUs.

To study the acquisition time of the BCH beacon, a simple geometricarrangement based upon path-loss shows that a WTRU at a cell's border,(equally distant between two neighboring BSs), experiences an intra-cellinterference (Ior) to inter-cell interference (Ioc) ratio (Ior/Ioc) of 0dB. The intra-cell interference (Ior) is the total received signal powerin a time slot from the BS in which the WTRU is communicating. Theinter-cell interference (Ioc) is the sum of the total received signalpower in the same TS from all the neighboring BSs. Intra-cellinterference (Ior) is therefore the “useful” energy, or the signal fromthe BS with which the WTRU is communicating. Inter-cell interference isthe interference caused by all the undesired signal energy from all theother BSs received by the WTRU, and is therefore detrimental to thedecoding of the “useful” signal.

This Ior/Ioc ratio is an important parameter for the performance of amultiuser detector (MUD). The analogous ratio which is found in the moreclassic signal detectors, such as RAKE receivers in frequency divisionDuplex (FDD) is Ec/Io, where “Ec” is the energy per chip of the desiredspreading code and Io is the sum of the energies of all other spreadingcodes which the WTRU can receive, but does not need to decode. As thegeometric path-loss situation is extended to more than just the twoclosest neighbors, the Ior/Ioc ratio will continue to decrease andapproach −1.5 dB.

Shadowing and fading will make the communications worse and moresporadic. In fading environments, it is anticipated that an Ior/Ioc inthe range of at least −1 to 0 dB or higher is needed in order to decodethe BCH properly with reasonable acquisition time. Analysis has shownthat for a time-aligned BCH TS communication system, approximately 25%of WTRUs at cell borders and 15% within all cell areas will experiencean Ior/Ioc <−1 dB. This results in a very degraded BCH beacon detectionwhich leads to detrimental effects on the user's perception ofquality-of-service. As the Ior/Ioc value decreases, the BCH beaconacquisition is compromised and WTRU synchronization under the worst-casecircumstances would be impossible for a significant part of thedeployment area.

It is therefore desirable to provide a novel beacon TS utilizationapproach to obviate the disadvantages discussed above.

SUMMARY

The present invention improves acquisition performance of beaconchannels using time-staggered beacon time slots. The present inventionmay be incorporated in a wireless multi-cell communication system, suchas a TDD system, which includes at least one WTRU and a plurality ofbase stations which broadcast beacon signals into a plurality of sectorsof a cell of the system. Beacon channel acquisition performance isenhanced by reducing beacon channel mutual interference. Each of thebase stations broadcasts a beacon signal into at least one of the cellsectors in a first time slot into at least another one of the cellsectors in a second time slot different from the first time slot. TheWTRU determines, at a particular position, which one of the beaconsignals has the best signal quality, acquires the one beacon signal andestablishes communications via the base station that generated the onebeacon signal having the best signal quality.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of preferred embodiments, given by way of exampleand to be understood in conjunction with the accompanying drawingwherein:

FIG. 1 is an illustration of an example showing a typical time slot ofthe prior art with one allocated beacon TS;

FIG. 2 is an illustration of an example showing cell sectors with theminimum of two allowed beacon TSs; and

FIG. 3 is an illustration of an example showing three sectored cells percell site.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention employs an interference avoidance strategy whichcan be used to improve the beacon channel acquisition performance. Thepresent invention can be implemented in a TDD system, or any othersystem whose cells can tolerate time staggering of the beacon channels.It can be deployed in an area using sectored cell sites or non-sectoredcell sites as well. In general, it is the characteristic of the cellsand their sectors, irrespective how the operator places its basestations to provide service. Although the figures herein only showsectored cells, it is also possible that one base station corresponds toone cell or sector when placed approximately in the middle of the cellor sector.

FIG. 1 illustrates a typical beacon TS14 of the prior art. The beacon TS14 is part of a frame 12. In wideband TDD, a frame comprises 15timeslots and has a 10 ms length, but in other systems a different typeof frame structure may be employed. For example, in narrowband TDD agroup of 7 timeslots with an overall of 5 ms length is called asub-frame. Two sub-frames make up a radio frame 10 having a length of 10ms, which comprises a frame of the multi-frame structure of the radioframe 10.

In the present invention, the beacon TSs are allocated to sectorizedcells such that their mutual interference is minimized according tocertain reuse patterns. The optimum reuse pattern depends on the numberof sectored or normal cell sites, the number of beacon TSs available andthe characteristics of the radio environment. Although the followingdescription refers to a TDD system as an example, the TDD system hasbeen selected for convenience. Accordingly, the present invention shouldnot be construed as being limited to a TDD system.

The time staggered approach could comprise a simple approach, whereinthe prior art allocated the beacons of all cells or sectors into TS(k)and TS(k+n) in every frame. The present invention proposes that a firstgroup of these cells or sectors having their Beacons in TS(k) andTS(k+n) in every frame and a second group of these cells or sectorshaving their beacons in TS(m) and TS(m+p) in every frame or anyextension of this timeslot reuse pattern principle. The UTRA TDDstandard does not fix the beacon to be in any particular timeslot(unlike GSM). It states that the beacon channel (BCH) must be at someTS(k), where k ranges from 1 to 7. When there is a second beacon TS inthe frame, it must be at TS(k+8).

A first embodiment of the present invention is shown in FIG. 2, whichutilizes a beacon timeslot reuse factor of two without spatial dimensioncell sectoring. The coverage area 28 has four base stations, BS1, BS2,BS3 and BS4. Each base station BS1-BS4 is assigned a different staggeredtime slot for its beacon transmission into one of three cells. The firstgroup of BSs transmit their beacons into cells 20 and 22 at time slotsTS(k) and TS(k+n) respectively. The second group of BSs transmits theirbeacons into cells 24 and 26, at timeslots TS(m) and TS(m+p),respectively. TS(k) represents a time slot at time k and TS (k+n)represents another time slot that is offset from TS(k) by n time slots.The same methodology follows through for cells TS(m) and TS(m+p).

A WTRU in position 29 becomes activated while it is located in the cell24 of BS2, which is in the coverage area 28. In this deploymentscenario, the WTRU can receive one of four possible time slot beacons,TS(k), TS(k+n), TS(m) or TS(m+p), each representing a possible celllocation. For this example and simplicity, the received beacon with thebest signal quality is the one being transmitted by BS2 into cell 24with a beacon time slot of TS(m). The WTRU in position 29 would acquirethis BCH beacon and establish its communication via BS2. As the WRTUmoves throughout the coverage area 28, it can monitor other beacons inthe different time slots and make a determination whether it should hopto another BS based on the beacon channel signals.

If the WTRU were to move into position 21 and the surrounding basestations BS1-BS4 were all transmitting their BCH beacon in the same timeslot, the WTRU would find it difficult to acquire a BCH beacon due tothe aforementioned Ior/Ioc ratio problems. Therefore, a reduction of themutual interference of the beacon channels of neighboring cells willimprove the average experienced Ior/Ioc for mobiles in the coverage areaand therefore improve BCH acquisition time.

In an alternative embodiment, a plurality of cells using cellsectorization or spatial dimensioning with three sectors per cell andantennas with a reuse of two is shown in FIG. 3. Each base stationBS1-BS4 broadcasts beacon signals into three sectors. The three sectorsare comprised of either two TS(k)s sectors and one TS(m) sector or oneTS(k) sector and two TS(m)s sectors. When BS1 transmits the BCH beaconinto sectors 31 or 33, the BS1 broadcasts the beacon at time slot k.Alternatively, when BS1 is transmitting into sector 32, the BS1transmits the beacon in time slot m.

Furthermore, besides the standard macro-cell sites, there are additionalembodiments for special cell sectorization for which the inventivemethod is applicable. For example, a pico-BTS cell for a large gatheringof people in a dense place, such as a sporting event or a conventioncould have sectors by rows of seats in the auditorium. A micro-BTS cellmay cover a street with two different sectors, one representing a northside of the street and the other sector representing the south side.

The present invention provides a very simple, yet effective interferenceavoidance strategy for beacon TS deployment in systems such as aUTRA-TDD system which can improve beacon acquisition time by means ofradio network planning. Applying the present invention with antennasector reuse cells enables UTRA-TDD system designers in dense urbanpedestrian WTRU environments the ability to create systems with anacceptable user system acquisition perception as well as pragmatic use.This invention is fully compliant with the UTRA UMTS standards,especially since mobiles are required to be able to measure neighboringcells on any time slot. It should be noted that beacon TS allocation tocells in the present invention and method can still be changed duringsystem operation and can be made especially effective when sectored cellsites are available.

The present invention described above, is not intended to replace thetime-aligned beacon TS approach which has advantages in terms of WTRUbattery life and system capacity. However, for deployment scenarioswhere a time-aligned beacon time slot deployment leads to unacceptablebeacon acquisition performance for a significant percentage of WTRUs inthe coverage area, the present invention represents a valuable andeasy-to-implement methodology for radio resource management (RRM).

Although particular processing functions have been described as beingperformed by particular components, it should be understood thatperformance of processing functions may be distributed among networkcomponents as desired.

Although the present invention has been described in detail, it is to beunderstood that the invention is not limited thereto, and that variouschanges can be made therein without departing from the spirit and scopeof the invention, which is defined by the attached claims.

1. A method implemented in a wireless multi-cell communication systemincluding a plurality of cells and a plurality of base stations, whereineach cell has at least three cell sectors, the method comprising: eachbase station broadcasting a beacon signal into two of the at least threecell sectors in a first time slot; and each base station broadcasting abeacon signal into the remaining cell sector(s) in a second time slotdifferent from the first time slot.
 2. The method of claim 1 furthercomprising: determining, at a particular position, which one of thebeacon signals has a best signal quality; and acquiring the one beaconsignal and establishing communications via the base station thatgenerated the one beacon signal having the best signal quality.
 3. Themethod of claim 2 wherein the determining and the acquiring areperformed by at least one wireless transmit/receive unit (WTRU) locatedat the particular position.
 4. The method of claim 1 wherein thewireless multi-cell communication system is a time division duplex (TDD)system.
 5. The method of claim 1 wherein each of the time slots isoffset from each of the other time slots by a predetermined number oftime slots.